1. Field of the Invention
The present invention relates to an amplifier circuit, and especially to a complementary metal-oxide semiconductor (CMOS) class AB amplifier.
2. Description of Related Art
FIG. 4 shows a conventional class AB amplifier, which includes an input-stage formed by a differential amplifier 41, and an output stage formed by an class AB amplifier 42, so as to perform a differential operation on an input signal, and amplify and drive the same for output.
The AB class amplifier can be analyzed as follows: The class AB amplifier 42 with a loading capacitor CL has an open loop voltage gain, from node A to node O, expressed as:                                                         A              AO                        ⁡                          (              s              )                                =                                                                      V                  O                                ⁡                                  (                  s                  )                                                                              V                  A                                ⁡                                  (                  s                  )                                                      =                                          (                                                      gm                    2                                    +                                                            gm                      1                                        ⁢                                          gm                      6                                        ⁢                                                                  r                        B                                                                                                                          sC                            B                                                    ⁢                                                      r                            B                                                                          +                        1                                                                                            )                            ⁢                              (                                                      r                    O                                                                                                      sC                        L                                            ⁢                                              r                        O                                                              +                    1                                                  )                                                    ,                            (        1        )            
where rB=rds4//rds5 is a DC resistance on node B; ro=rds2//rds6 is an DC output resistance of the output stage; gmj represents transfer-conductance of a MOS transistor Mj, j=1,2,3 . . . ; CB=CDB4+CDB5+CGD4+CGD5+gm6rOCGD6 is a parasitic capacitance of the output MOS transistor M6 and the current mirror amplifier consisting of MOS transistors M4 and M5.
When the class AB amplifier employs in an operational amplifier with a compensation capacitor CF, its output impedance with a loading capacitor CL becomes:                                                         y              O                        ⁡                          (              s              )                                =                                    1              /                                                z                  O                                ⁡                                  (                  s                  )                                                      =                                                                                                      sC                      F                                        ⁢                                          r                      A                                                        +                  1                                                  r                  A                                            +                                                                                          sC                      F                                        ⁢                                          r                      A                                                                                                                          sC                        F                                            ⁢                                              r                        A                                                              +                    1                                                  ⁢                                  (                                                            gm                      2                                        +                                                                  gm                        1                                            ⁢                                              gm                        6                                            ⁢                                                                        r                          B                                                                                                                                    sC                              B                                                        ⁢                                                          r                              B                                                                                +                          1                                                                                                      )                                            +                              1                                  r                  O                                            +                              sC                L                                                    ,                            (        2        )            
where rA is a DC resistance on node point A, i.e., an output resistance of the differential amplifier. If xcfx89 greater than  greater than 1/(rACF), and
            gm      2        ⪢                  1                  r          A                    +              1                  r          O                      ,
we have                                                         y              OH                        ⁡                          (              s              )                                =                                    1              /                                                z                  OH                                ⁡                                  (                  s                  )                                                      =                                          gm                2                            +                                                gm                  1                                ⁢                                  gm                  6                                ⁢                                  r                  B                                ⁢                                  1                                                                                    sC                        B                                            ⁢                                              r                        B                                                              +                    1                                                              +                              sC                L                                                    ,                            (        3        )            
herein, the term       gm    1    ⁢      gm    6    ⁢      r    B    ⁢      1                            sC          B                ⁢                  r          B                    +      1      
represents a gyrated output inductor,
where,
LOEQ≈CB/(gm1gm6),xe2x80x83xe2x80x83(4)
and with an equivalent series resistor:
RSEQ=1/(gm1gm6rB).xe2x80x83xe2x80x83(5)
Therefore, we have                                                         z              OH                        ⁡                          (              s              )                                =                                                                      sr                  B                                ⁢                                  C                  B                                            +              1                                                                        s                  2                                ⁢                                  r                  B                                ⁢                                  C                  L                                ⁢                                  C                  B                                            +                              sC                L                            +                                                sgm                  2                                ⁢                                  r                  B                                ⁢                                  C                  B                                            +                                                gm                  1                                ⁢                                  gm                  6                                ⁢                                  r                  B                                            +                              gm                2                                                    ,                            (        6        )            
Due to gm1gm6rB greater than  greater than gm2, zO(s) resonates at:                                           ω            R                    =                                                                                                                                        gm                        1                                            ⁢                                              gm                        6                                            ⁢                                              r                        B                                                              +                                          gm                      2                                                                                                  r                      B                                        ⁢                                          C                      B                                        ⁢                                          C                      L                                                                                  ≈                                                                    1                                                                  r                        B                                            ⁢                                              C                        B                                                                                            ⁢                                                                                                    gm                        1                                            ⁢                                              r                        B                                            ⁢                                              gm                        6                                                                                    C                      L                                                                                            =                                                                                                      gm                      1                                        ⁢                                          gm                      6                                                                                                  C                      B                                        ⁢                                          C                      L                                                                                  =                              1                                                                            L                      OEQ                                        ⁢                                          C                      L                                                                                                          ,                            (        7        )                                                      Q            R                    =                                                                      r                  B                                                                      C                    L                                    +                                                            gm                      2                                        ⁢                                          r                      B                                        ⁢                                          C                      B                                                                                  ⁢                                                                    gm                    1                                    ⁢                                      gm                    6                                    ⁢                                      C                    B                                    ⁢                                      C                    L                                                                        =                                                                                                      gm                      2                                        ⁢                                          r                      B                                        ⁢                                          C                      B                                        ⁢                                          C                      L                                                                                                            C                    L                                    +                                                            gm                      2                                        ⁢                                          r                      B                                        ⁢                                          C                      B                                                                                  ⁢                                                                                          gm                      1                                        ⁢                                          r                      B                                        ⁢                                          gm                      6                                                                            gm                    2                                                                                      ,                            (        8        )            
and has a resonated impedance:                               "LeftBracketingBar"                                    z              OH                        ⁡                          (                              j                ⁢                                  xe2x80x83                                ⁢                                  ω                  R                                            )                                "RightBracketingBar"                =                              (                          1                                                C                  L                                +                                                      gm                    2                                    ⁢                                      r                    B                                    ⁢                                      C                    B                                                                        )                    ⁢                                                                      C                  B                                ⁢                                  C                  L                                                                              gm                  1                                ⁢                                  gm                  6                                                              ⁢                                                                                                                gm                      1                                        ⁢                                          gm                      6                                        ⁢                                          r                      B                      2                                        ⁢                                          C                      B                                                                            C                    L                                                  +                1                                      .                                              (        9        )                        If      ⁢              xe2x80x83            ⁢                                    gm            1                    ⁢                      gm            6                    ⁢                      r            B            2                    ⁢                      C            B                                    C          L                      ⪢    1    ,
we have:                                           "LeftBracketingBar"                                          z                OH                            ⁡                              (                                  j                  ⁢                                      xe2x80x83                                    ⁢                                      ω                    R                                                  )                                      "RightBracketingBar"                    ≈                                                    r                B                            ⁢                              C                B                                                                    C                L                            +                                                gm                  2                                ⁢                                  r                  B                                ⁢                                  C                  B                                                                    ,                            (        10        )            
if gm2rBCB greater than  greater than CL, we have                               "LeftBracketingBar"                                    z              OH                        ⁡                          (                              j                ⁢                                  xe2x80x83                                ⁢                                  ω                  R                                            )                                "RightBracketingBar"                ≈                              1                          gm              2                                .                                    (        11        )            
From the above analysis, it is known that the compensation capacitor CF in the operational amplifier will cause a gyration effect, so that the internal parasitic capacitance CB=CDB4+CDB5+CGD4+CGD5+gm6rOCGD6 is gyrated to be the inductance in the output impedance. Therefore, the resonated output impedance |ZOH(jxcfx89R)| always induces a peak gain on a operational amplifier around several 1 MHz to several 10 MHz. As a result, the gain margin of the operational amplifier is insufficient, or even becomes negative, which will cause a serious problem of oscillating. In general, to avoid this problem, it is necessary to design a very low gain margin for the differential amplifier at the output stage of the operational amplifier. However, such a design will cause various problem such as greatly decreasing the gain bandwidth and increasing the DC offset.
In Japan Patent publication No. 09-01823, there is disclosed an operational amplifier circuit which may alleviate the aforementioned problem by reducing the equivalent value of rB. However, such a circuit will cause a problem of insufficient output driving ability, and thus the improvement is limited.
Therefore, it is desirable for the above conventional circuit to be improved so as to mitigate and/or obviate the aforementioned problems.
Accordingly, the object of the present invention is to provide a complementary metal oxide semiconductor class AB amplifier for achieving the purposes of saving power and having a large driving capability without sacrificing the gain bandwidth.
In accordance with one aspect of the present invention, there is provided a CMOS class AB amplifier having an adaptive level shift circuit, a compensation capacitor and an output transistor pair to receive an output of a differential amplifying circuit for being provided with a DC bias and a DC amplifying effect by the adaptive level shift circuit, phase compensated by the compensation capacitor, and driven by the output transistor pair. The adaptive level shift circuit comprises: a current mirror circuit having a first NMOS transistor and a second NMOS transistor, each having a gate connected to one another; a NMOS diode transistor having a gate and a drain connected to the gate; a NMOS switch transistor having a source connected to a drain of the second NMOS transistor of the current mirror circuit, a drain connected to a drain of the first NMOS transistor of the current mirror circuit and a source of the NMOS diode transistor, and a gate connected to the gate and drain of the NMOS diode transistor; a PMOS input amplifier transistor having a drain connected to the drain of the NMOS diode transistor; and, a PMOS current source transistor having a drain connected to the source of the NMOS switch transistor.
In accordance with another object of the present invention, there is provided a CMOS class AB amplifier having an adaptive level shift circuit, a compensation capacitor and an output transistor pair to receive an output of a differential amplifying circuit for being provided with a DC bias and a DC amplifying effect by the adaptive level shift circuit, phase compensated by the compensation capacitor, and driven by the output transistor pair. The adaptive level shift circuit comprises: a current mirror circuit having a first PMOS transistor and a second PMOS transistor, each having a gate connected to one another; a PMOS diode transistor having a gate and a drain connected to the gate; a PMOS switch transistor having a source connected to a drain of the second PMOS transistor of the current mirror circuit, a drain connected to a drain of the first PMOS transistor of the current mirror circuit and a source of the PMOS diode transistor, and a gate connected to the gate and drain of the PMOS diode transistor; a NMOS input amplifier transistor having a drain connected to the drain of the PMOS diode transistor; and, a NMOS current source transistor having a drain connected to the source of the PMOS switch transistor.